So did they just... remove Facebook login? Doesn't seem that likely. Maybe the FB SDK has some flags you can set.
It sounds like this they're just no longer flat out using the Facebook SDK (which provides a slightly more "native" / "nicer" login flow for apps when used). They're going to do what most (at least, from personal experience) apps do and just show a webview with a redirect back into the app, which doesn't call out to Facebook at all.
EDIT: That is, it doesn't call out to Facebook at all until you start the login flow, which is just opening a browser view to the oauth2 flows..